The invention relates to a single-phase synchronous motor comprising a two-pole permanent-magnet rotor between two electromagnetically excited stator poles.
A two-pole single-phase synchronous motor comprising a permanent-magnet rotor is known from the magazine ETZ, Vol. 30, 1978, No. 2, pages 53 to 60. The rotor comprises a diametrally magnetized anisotropic permanent magnet which is sintered in one piece and which is formed with a bore through which the rotor shaft extends. The manufacture of such cylindrical high-grade permanent magnets formed with anaxial bore is difficult if the rotor length or the ratio between the rotor length and the rotor diameter exceeds specific values. The power ratings of these motors, which are generally dimensioned for short-time operation, therefore do not exceed 25 W.
From the article by Karl Ruschmeyer, "Motoren und Generatoren mit Dauermagneten", in Vol. 123 of the series "Kontakt und Studium", edited by Prof. Dr. Ing. Wilfried J. Bartz, and published by the Expert Verlag, 1983, pages 36 and 37, d.c. motors are known in which a soft-magnetic iron core is provided with hard-magnetic permanent magnets. The permanent magnets are arranged side by side at a distance from each other along the peripheral surface of the iron core. These permanent magnets form poles of alternate polarity. The soft-magnetic iron core is radially symmetrical. The permanent magnets are rare-earth magnets, which are expensive. The use of small magnet blocks is therefore advantageous in comparison with solid rotors of a rare-earth magnet material.
A further disadvantage of single-phase synchronous motors with permanent-magnet rotors as described in ETZ is that these motors exhibit comparatively large fluctuations of the instantaneous angular velocity with a frequency of 100 Hz, which fluctuations may have amplitudes of more than .+-.30% of the synchronous angular velocity. These fluctuations may be attributed to the alternating torque which is inevitable during pure single-phase operation and to the magnetic detect torque. In order to improve the synchronious operation it is known from DE-AS No. 14 88 267 to compensate for the alternating torque by means of an additional magnet which rotates in a separate magnetic circuit. However, this method is intricate and results in a larger motor volume.
When high-grade magnet materials with a high remanent induction are used, as in the case of rare-earth materials, starting is affected by transient oscillations (DE-PS No. 34 03 041). Moreover, the starting voltage at which a correct starting of the motor is ensured increases. In accordance with DE-PS No. 34 03 041 transient oscillations can be avoided, in that the moment of inertia and the air gap, which also determines the detent torque, are influenced in such a way that a natural frequency of the system which comprises the rotor and the load and which oscillates freely with a low amplitude is unequal to the input voltage frequency.